Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, and especially relates to a configuration of a sheet detector that detects passage of a rear end of a sheet.
Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, a printer, or a facsimile machine, includes a sheet conveying apparatus that conveys a sheet. With the sheet conveying apparatus, the image forming apparatus conveys the sheet to an image forming portion, transfers a toner image formed on a photosensitive drum to the sheet, conveys the sheet on which the toner image is transferred to a fixing portion, and then conveys the sheet to a sheet discharging portion. In recent years, the image forming apparatus is required for further improvement of productivity, that is, improvement of the number of image formed sheets per unit time. Therefore, speeding up of a sheet conveying speed and reduction of an interval from a rear end of a sheet to a tip of a subsequent sheet of continuously conveyed sheets (hereinafter, referred to as sheet interval) have been achieved.
By the way, when the sheet is conveyed in the conventional sheet conveying apparatus, a switching operation of various switching portions and a switching operation of a rotating direction of a sheet conveying portion are performed based on detection of the rear end of the sheet. For the detection of the rear end of the sheet, a sheet detector that detects the rear end of the sheet is provided on a sheet conveying path.
Here, as the sheet detector, there is one that includes a sensor flag (see Japanese Patent Laid-Open No. 6-94444). The sensor flag performs reciprocating motion between a standby position and an allowable position where passage of the sheet is allowed, by being pressed by the sheet every time the sheet passes through. However, the sheet detector having the configuration of allowing the sensor flag to perform reciprocating motion and detecting the passage of the sheet cannot detect the passage of the sheet if the sheet interval is short, because the sensor flag requires a certain time to return from the allowable position to the standby position.
Therefore, a sheet detecting apparatus is disclosed (see U.S. Patent Application Publication No. 2011/089629 A1), which includes a sensor flag including a plurality of abutting pieces that abuts on the sheet, in the same direction to a conveying direction of the sheet in a rotatable manner so that the passage of the sheet can be detected even if the sheet interval is short. In this sheet detecting apparatus, when the sheet is conveyed, one of the plurality of abutting pieces is pressed by the sheet, and the sensor flag is rotated in the sheet conveying direction. Then, when the rear end of the sheet passes through the sensor flag, other abutting pieces are sequentially moved to the standby position where the abutting piece abuts on a subsequently conveyed sheet. Accordingly, the sheet detecting apparatus can detect the passage of the rear end of a preceding sheet, and can prepare for detection of a tip of a subsequently conveyed sheet. Therefore, the sheet detecting apparatus can support speeding up of the sheet conveying speed and a shorter sheet interval than before.
By the way, there is a case in which the conventional sheet conveying apparatus conveys a sheet having extremely low rigidity, such as a thin paper having a basis weight of less than 60 g/m2. In this case, when the rear end of the preceding sheet passes through the sensor flag, the abutting piece of the sensor flag may sometimes be moved to the standby position while pushing the rear end of the sheet aside. In this case, timing when the sensor flag is moved to the standby position becomes earlier than timing of a case of a sheet having higher rigidity, and timing of the detection of the rear end of the sheet becomes earlier. As described above, when there is difference in the rigidity of the conveyed sheets, a difference is caused in the timing of the detection of the rear end of the sheet.
Here, a detection signal for detecting the rear end of the sheet in control of conveying the sheet is used as a reference for the switching operation of various switching portions and the switching operation of a rotating direction of a conveying portion. Therefore, if the difference is caused in the timing of the detection of the rear end of the sheet depending on the difference of the rigidity of the sheet, the switching timing needs to be set with a temporal margin in expectation of detection variation, and this may hinder improvement of the productivity.
When the rear end of the sheet passes through, the sensor flag is moved to the standby position by a positioning mechanism that includes a spring and a cam. Therefore, if the spring for moving the sensor flag is changed to a spring having a small spring force, occurrence of the difference in the timing of the detection of the rear end of the sheet depending on the difference of the rigidity of the sheet can be prevented. However, when the spring having a small spring force is used, the spring cannot stop the sensor flag that continues the rotation by inertia, at the standby position.
Therefore, the present invention has been made in view of the foregoing, and it is desirable to provide a sheet conveying apparatus and an image forming apparatus that can accurately detect a position of a rear end of a sheet regardless of rigidity of the sheet.